1. Field of the Invention
The present invention relates to a protection circuit applied to wind power generation system employing a double-fed induction generator.
2. Description of the Related Art
In general, a wind power generation system sometimes utilizes a double-fed induction generator. The double-fed induction generator includes a stator and a rotor. The stator is directly connected to a grid (an electric power system). The rotor is connected to a converter (an electric power converter) which excites the rotor. This rotor is rotated by wind force. According to such a configuration, the wind power generation system generates an electric power by the wind force and supplies the electric power to the grid.
In this wind power generation system, a grid voltage lowers sometimes when a grid fault occurs. When the grid voltage lowers, an overcurrent flows from the rotor into the converter. Furthermore, when the rotor is excessively speeded up by a wind gust, a voltage on the side of the rotor rises, so that the overcurrent flows from the rotor into the converter. Such a current brings about the overcurrent or overvoltage of the converter.
Thus, for protecting the converter from such an overcurrent, it is known that a protection circuit is provided in the wind power generation system (see e.g. Jpn. Pat. Appln. KOKAI Publication No. 2006-230085).
However, for continuously performing ride through of the wind power generation system upon such a grid fault (LVRT, low voltage ride through), as the protection circuit, a large-sized circuit is only known.